1. Field of the Invention
The present invention relates, in general, to satellite imagery and collecting image data from orbiting satellites such as geostationary satellites, and, more particularly, to methods and devices for correcting or improving satellite imagery or collected image data during the data collection stage so as to account for earth curvature and/or certain other effects that cause distortion or loss of resolution in an image.
2. Relevant Background
In recent years, there has been a growing demand for accurate imaging of the earth such as can be obtained from orbiting satellites. Each imaging satellite can be thought of as including an image or image data collection system that typically includes an optical system or other image collector and image sensors. For example, the satellite may be equipped with a telescope and one or more image sensors. Historically, earth imaging from a satellite has been performed using scanning radiometers, but two dimensional charge couple devices (2D CCDs) or focal plane arrays have become increasingly important for a wide variety of imaging applications, including earth observations and astronomical observations from satellites. For example, large CCD imaging sensors are currently used as the imaging sensors for the Hubble Space Telescope and in the Cassini planetary mission to Saturn and Titan. In general, 2D CCDs are simpler, faster, and have fewer moving parts than scanning radiometers. The emergence of 2D array imaging devices has the potential to significantly change earth observations by providing a greater emphasis on the resulting image and its properties.
Applications depending on space-based observing systems designed to observe large areas of the earth's surface or the atmospheric environment and clouds above the earth's surface are limited by the well known fact that the spatial resolution of imagery and geophysical sensing measurements from a satellite degrades with distance from nadir (i.e., the point in the sky directly below the observing satellite) or within the portion of the results approaching the edge of the visible earth or visible earth disk when considered as a 2D image. The loss of resolution at the edges of the image or distal to nadir is caused by a progressive foreshortening of the earth features in the image and a corresponding increase in the ground sample distances. From a satellite in low earth orbit (LEO), the foreshortening is primarily the result of oblique viewing angles coupled with increasing distances between the satellite and the earth features that are being observed. From a satellite in geostationary orbit (i.e., orbit about the earth's equator that makes the object appear motionless in the sky), issues associated with oblique viewing angles are compounded by earth curvature effects that become the dominant factor in the loss of image resolution in areas away from nadir. This loss of resolution can be so severe as to become the most important factor limiting the coverage areas for which quantitative use of the satellite observations is possible. While post-collection processing of the collected imagery can cosmetically “correct” an image for the distortion inherent in the initial observation, such remapping techniques cannot improve the inherent quality of the observation or its inherent resolution.
Earth curvature is thus the dominant factor causing loss of imaging resolution for many satellites due to a significant increase in the observation footprint or instantaneous field of view (IFOV). For example, an instrument or imaging device on a satellite in a geostationary orbit with an 8-kilometer IFOV at nadir may have a reduced resolution of 24 kilometers or worse as the instrument or device scans areas that are far from the equatorial regions at the center of the image. These edge effects are not the result of any defect in the satellite instrument or image collection system but are rather the result of the coupled geometry of the satellite and the spherical earth being studied. From an earth observation point of view, the decreased resolution proximate to the edge of the earth disk is an image defect that is typically axially symmetric around the sub-satellite point on the earth's surface. Such an image defect is readily apparent from the perspective of geostationary satellites but is also present in image data collected from satellites in other orbits.
To date, the only satellite-based observation system that has employed techniques to limit the inherent loss in image resolution towards the edge of an imaging swath is the Operational Linescan System (OLS) used by the U.S. Defense Military Satellite Program (DMSP). The OLS instrument is a scanning radiometer flown in low earth orbit (LEO). The OLS maintains its cross-track resolution by using a segmented scanning sensor that progressively turns off individual sensor segments as it points further away from nadir. In effect, the instrument reduces the physical size of its sensor element as it views the earth at increasingly oblique angles. While this approach succeeds in limiting the overall growth in the sensor footprint away from nadir, the achieved sensor resolution varies along its scanning path as sensor segments are turned off or on. This general approach is also being used for the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument proposed for inclusion in the NPOESS operational polar orbiting meteorological satellite system.
Hence, there remains a need, particularly for geostationary satellites, for methods and systems that address the inherent loss in image resolution caused by the curvature of the earth and other effects. In some cases, it may be preferable that specially designed optical systems designed to be used in conjunction with large focal plane array sensing devices be used to fulfill these needs. Similar optical devices may also provide an attractive alternative to the large, mechanically complicated scanning radiometers that are currently being employed to try to address image resolution loss in low earth orbit applications.